Fiber MOPAs, including frequency converted fiber MOPAs are increasingly being used in applications where frequency-converted solid lasers were previously used. Such applications include micro-machining/materials processing and wafer inspection. Fiber laser and fiber-amplifier systems have certain advantages over solid state lasers. These advantages include more efficient use of pump power, permanence of alignment, and in many instances a convenience of packaging which is due to the fact that amplifier fibers can be coiled in an enclosure.
A principal advantage of a fiber-amplifier is a high gain (increase of average power and pulse energy by a factor of up to 1,000) combined with a low saturation energy and low saturation power. High gain and low saturation energies allow the use of low power, pulsed seed lasers, which in turn opens up the parameters space of laser performance in terms of repetition rate and pulse duration. This is because pulse durations in the nanosecond and sub-nanosecond regime and repetition rates above several hundred kilohertz (kHz) can only be realized in micro-chip solid-state lasers and pulsed or modulated-CW diode lasers. Both of these laser types exhibit low output power, for example less than about 100 milliwatts (mW).
FIG. 1 schematically illustrates a general layout 10 of a prior-art fiber MOPA having a low-power seed source 12. Seed source 12 can be either a pulsed laser or a continuous wave (CW) laser. Pulsed lasers are usually mode-locked lasers providing short duration pulses (nanoseconds or less) at repetition rates of a few megahertz (MHz). Output of laser 12 is passed via an isolator 14 to one or more fiber preamplifier stages 16 which amplify the seed radiation to an average power of about 100 mW.
The pre-amplified radiation is delivered to a device 18, which establishes the output pulse repetition frequency (PRF) for MOPA 10. If the seed radiation is CW radiation, device 18 is a modulator such as an electro-optic (E-O) or an acousto-optic (A-O) modulator. If the seed radiation is pulsed radiation, device 18 is a pulse-picker which reduces the seed pulse PRF by selecting every Nth one of the input pulses and discarding the remainder. The PRF out of device 18 may be between about 10 kilohertz (kHz) and a several megahertz (MHz).
As a result of the modulation or pulse-picking the average power output of device 18 can be reduced to about 1 mW. After being passed through an isolator 22, this output is “re-amplified” by one or more fiber preamplifier stages 20. Several pre-amplifiers are usually used to increase the output power of amplifier stages 20 to about the single Watt level. The numbers of pre-amplifiers used depends on the average power of the seed source and the duty-cycle (PRF) reduction by device 18.
The lower the seed source average output power and the lower the final duty cycle, the more pre-amplifiers need to be incorporated into the amplifier chain. The output of amplifier stages 20 is then passed through an isolator 24 to a fiber power amplifier 26 for final amplification to an average power level of about 100 W.
Many implementations of the above described general fiber MOPA architecture have been published in the scientific literature, with pulse energies of up to 1 millijoule mJ, average output powers in excess of 100 W, repetition rates between 10 kHz and several MHz, and pulse durations between a few nanoseconds (ns) and hundreds of femtoseconds (fs). Publications include: C. Brooks et al, Optics Express 13(22), 8999, 2005; L. Shah et al, IEEE J. Quant. Electron. 22(3), 552, 2007; K-H. Liao et al., Optics Express 15(8), 4876, 2007; Y. Zaouter et al., Opt. Lett. 33(13), 1527, 2008; G. Chang et al, CLEO 2008; J. Limpert et al., IEEE J. Quant. Electron., 15(1), 159, 2009; and M. J. Leonardo et al., Proc. of SPIE, vol. 7195, 71950F-1, 2009.
A primary limitation of the Fiber MOPA architecture results from non-linear effects in the fibers. Due to a small core size of amplifier fibers, typically tens of micrometers (μm) in diameter, and a long length of the amplifier fibers (typically several meters), nonlinear effects such as self-phase modulation (SPM) and four-wave mixing lead to an increase in spectral bandwidth of amplified radiation. A narrow spectral bandwidth of typically less than 0.5 nm (FWHM) is required at the infrared signal wavelength to achieve efficient harmonic generation by doubling or sum frequency generation. In addition to these effects, stimulated Raman scattering (SRS) induces an energy transfer from the signal wavelength to a shifted wavelength. Stimulated Brillouin scattering, may lead to the partial reflection of the signal wave and destruction of the fiber system components.
In order to avoid spectral broadening effects pulse energy must be limited at any given pulse duration. By way of example, the pulse energy out of the Yb-doped fiber-amplifier is limited to less than about one microjoule μJ for pulse duration of less than 100 picoseconds (ps), and between about 1 and 20 μJ at longer pulse durations.
A common method of increasing the pulse energy limit is to increase the core diameter of the fiber-amplifier to up to about 40 μm by using amplifier fibers having a photonic crystal structure. Such fibers have regions of low effective refractive index of the core region, created by arrays of doped and undoped fiber sections. For core diameters between 40 and 100 um, such fibers are also referred to as rod fibers. Using this type of amplifier fiber enables access to pulse energies in excess of 1 mJ in the ns-pulse regime, and, combined with chirped pulse amplification, allows for generation of pulse energies of up to 100 μJ for ps and fs pulses. However, these photonic-crystal fibers are difficult to manufacture and, in addition, suffer from a power degradation process referred to by practitioners of the art as photo-darkening. Chirped-pulse amplification involves the cost and space required for grating based pulse stretchers and compressors.
Another disadvantage of rod-type power-amplifier fibers is that they are relatively inflexible, and very difficult to package in a convenient space. In a relatively low power frequency-converted MOPA system, for example having an average power for fundamental radiation of less than about 50-100 Watts (W), the master oscillator, fiber-amplifier stages, diode-laser arrays for providing optical pump radiation, and one or two stages of harmonic conversion can usually be packaged in a single enclosure having a “footprint” of about 60 centimeters (cm)×20 cm. Power for powering the diode-lasers and other components can be supplied to the enclosure from a separate power supply, via a suitable fiber-cable and electrical connectors. Adding a power amplifier stage comprising a rod-type fiber will require a much greater packaging space. This is because the power-amplifier fiber will have a large bending radius imposed either by mechanical inflexibility or by bending losses due to a low numerical aperture of the fiber.
There is a need for fiber high-power MOPA architecture that enables output energies above microjoules and up to Joules; does not require chirped pulse amplification; is not subject to degradation by photodarkening, and which can be contained in a package having about the same footprint as a prior-art art low-power MOPA. Preferably this should be accomplished while preserving, as much as possible, above described advantages, of fiber MOPAs that make such fiber MOPAs, preferable to solid state MOPA's.